The present invention relates generally to demodulators for digital radio communications systems, and more specifically to interference cancellation for cancelling interfering signals from undesired multiple sources.
According to a prior art interference cancellation technique as described in Japanese Patent No. 62-16580, an auxiliary antenna is employed for detecting undesired signal, which is supplied to a correlator and an amplitude and phase controller to produce a cancelling signal. Desired signal is detected by the mainlobe of a main antenna. The output of the main antenna is fed to a summing amplifier where it is combined with the cancelling signal to cancel the undesired signal which is also detected by a sidelobe of the main antenna. To derive the cancelling signal, correlation is detected between the outputs of the main and auxiliary antennas by the correlator and used as an input to the controller to modify the output of the auxiliary antenna both in amplitude and phase. Being derived from correlation between the undesired signals detected by the main and auxiliary antennas, the signal-to-noise ratio of the interference cancelling signal is not sufficient to cancel interference, particularly when it is desired to cancel interfering signals coming from multiple sources. Further, in cases where the desired signal is also detected by a sidelobe of the auxiliary antenna, the correlation between it and the desired signal on the main path will be detected by the correlator. Such correlation would act in a way to decrease the desired signal.